High melting N,N-terephthaloyl bis-phthalimide and its use as an ester interlinking agent for polyesters

ABSTRACT

There is disclosed, as a composition of matter, a new, high melting N,N&#39;-terephthaloyl bis-phthalimide, a method for its preparation employing as a solvent a high boiling aromatic or aliphatic-aromatic ether or a mixture of a high boiling aromatic or aliphatic-aromatic ether and a low boiling cyclic or aliphatic ether and the use of said new, high melting N,N&#39;-terephthaloyl bis phthalimide as an ester interlinking agent for polyester.

This invention relates to N,N'-terephthaloyl bis-phthalimide. Moreparticularly the invention relates to a method of preparingN,N'-terephthaloyl bis-phthalimide having a high melting point, to itsuse as an interlinking agent in the preparation of high molecular weightpolyesters and copolyesters and to fibers and other molded productsthereof.

U.S. Pat. No. 2,558,675 discloses the preparation of polyimidederivatives of carboxylic acids by the condensation of an acid imide(preferably in the form of an alkali metal salt) with a polyacylchloride or polyacyl bromide of a carboxylic acid. This prior artpatent, as well as U.S. Pat. No. 2,594,145, further discloses that saidderivatives find utility in the preparation of highly polymericmaterials by using the derivatives to join or interlink molecules ofmoderate molecular weight. An attempt was made to repeat the teachingsof U.S. Pat. No. 2,558,675, particularly with respect to the preparationand use of N,N'-terephthaloyl bis-phthalimide. The procedure set forthin Example 5 (see Column 4, lines 4-15) of the above cited patent wasfollowed as closely as possible. There was obtained a product having amelting point (270°- 275° C. with decomposition) corresponding to thatreported for the product in Example 5 of the above noted patent.Unexpectedly, however, and contrary to the disclosure of the patent,this product, when added to a melted sample of polyester caused nointerlinking of the chain ends. In fact, the polyester sample actuallyunderwent a reduction in molecular weight as evidenced by a decrease inits original intrinsic viscosity. Repeated attempts at usingrecrystallized samples of the product failed to provide any differentresult. Thus it was found that the product prepared according to theteachings of Example 5 of U.S. Pat. No. 2,558,675 does not promote theester interlinking reaction of polyester chains.

Now, in contrast to the teaching of U.S. Pat. No. 2,558,675, it has beenfound that when a mixture of terephthaloyl chloride and an imidecompound selected from phthalimide and alkali metal salts of phthalimidesuch as lithium phthalimide, sodium phthalimide, potassium phthalimideand the like are reacted in high boiling aromatic or aliphatic-aromaticethers or mixtures comprising a high boiling aromatic oraliphatic-aromatic ether component and a low boiling cyclic or aliphaticether component the product has a melting point of at least 330° C.Furthermore, this product has ester interlinking properties and iseffective in increasing the polymerization rate of polyester formingreactants as well as increasing the molecular weight of condensationpolyesters.

In order to prepare the new high melting N,N'-terephthaloylbis-phthalimide described herein in good yield the reaction between theterephthaloyl chloride and the phthalimide or alkali metal salt thereofis carried out at temperatures ranging from 70° to 170° C. in theinitial stages of the reaction and at temperatures ranging from 200° to400° C. in the final stages of the reaction.

Preferably these conditions are accomplished through the use of asolvent medium consisting initially of a mixture of a high boiling ethercomponent selected from the group consisting of aromatic andaliphatic-aromatic ethers and a low boiling ether component selectedfrom the group consisting of aliphatic and cyclic ethers. The high andlow boiling ether components are selected such that during the initialreaction stages (i.e., the first 0.5 - 5.0 hours) in the preparation ofthe high melting N,N'-terephthaloyl bis-phthalimide the temperature ofreaction will fall within the above specified 70°- 170° C. range. As thereaction proceeds the low boiling ether component is slowly distilledout of the reaction mixture and the preparation continued to completionat a temperature between 200°- 400° C. and preferably at the boilingpoint of the high boiling ether component. Given the respective boilingpoints of any particular high boiling and low boiling ether one ofordinary skill in the art can readily determine the amounts of eachwhich will be needed to produce a solvent mixture to provide an initialreaction temperature within the limits specified above. For example, inthe preparation of high melting N,N'-terephthaloyl bis-phthalimidedescribed in Example 1 below the solvent mixture employed consisted ofdiphenyl ether having a boiling point of 254° C. and dioxane having aboiling point of 101° C. It was readily determinable that a 50/50 volumemixture of these two ethers would provide for a reaction temperaturebetween the prescribed 70° - 170° C. range. The actual boiling point ofthe mixture was observed to range from 110° - 115° C.

The high melting N,N'-terephthaloyl bis-phthalimide of this inventioncan also be prepared employing the high boiling ethers, describedhereinbelow, as the sole solvent medium. Again, as with the use of themixed solvent system, the initial stages of the reaction, i.e., thefirst 0.5 to 5.0 hours of the reaction, are carried out at temperaturesranging from 70° to 170° C. For the final stages of the reaction thetemperature is raised to between 200° and 400° C. and preferably to theboiling point of the high boiling ether solvent to complete thereaction. Failure to observe the above temperature conditions, andparticularly the initial temperature conditions when employing a highboiling ether as the sole solvent medium, will lead to poor conversionsdue to volatilization of the terephthaloyl chloride reactant.

By the term "high boiling" ether component is meant those aromatic andaliphatic aromatic ethers having boiling points ranging from 200° to400° C. and melting points below 50° C. and preferably below 30° C.While it is most convenient to employ high boiling ethers which areliquids at ambient temperatures the most preferred high boiling ether,diphenyl ether, is a solid at ambient temperatures which must be meltedprior to use. By the term "low boiling" ether component is meant thosealiphatic and cyclic ethers having boiling points between 65° and 150°C.

In addition to diphenyl ether, other examples of useful high boilingethers include benzyl butyl ether, butyl phenyl ether, isoamyl phenylether, hexylphenyl ether, heptylphenyl ether, octylphenyl ether, propyltolyl ether, butyl tolyl ether, methyl naphthyl ether, ethyl naphthylether, propyl naphthyl ether and the like. Examples of useful lowboiling ethers, in addition to dioxane, include isopropyl ether, ethylisobutyl ether, ethyl isoamyl ether, isobutyl ether, ethylhexyl ether,butyl ether, tetrahydrofuran, tetrahydropyran and the like.

According to the preferred procedure for preparing the high meltingN,N'-terephthaloyl bis-phthalimide disclosed herein, terephthaloylchloride and potassium phthalimide are reacted in a mixed solvent systemconsisting of 50 percent by volume diphenyl ether and 50 percent byvolume of dioxane. Employment of this particular mixed solvent systemprovides reaction temperatures between 110° to 115° C. during theinitial one to two hours of the preparation. During the course of thereaction and after the initial 1 to 2 hours of reaction time the dioxaneis gradually distilled from the reaction mixture and the reactioncompleted at the boiling point of the diphenyl ether over an additional1 to 2 hour period. Thus, during the course of the preparation thereaction temperature ranges from about 110° C. to 254° C. At the end ofthis time the reaction mixture is filtered to remove the potassiumchloride by-product and then cooled and the N,N'-terephthaloylbis-phthalimide precipitating from the diphenyl ether at temperaturesdown to 60° C. collected and dried.

The following examples illustrate the invention. In these examples allparts and percentages are by weight unless otherwise indicated. "I.V."stands for intrinsic viscosity as measured in a 60/40phenol/tetrachloroethane mixed solvent at 30° C. and is a measure of themolecular weight of the polyester.

EXAMPLE 1

The apparatus employed to prepare the high melting N,N'-terephthaloylbis-phthalimide consists of a 2000 milliliter (ml.) glass reactionvessel equipped with a stirrer, thermometer, condenser, nitrogen inletand a valved outlet located at the bottom of the reaction vessel. Theglass reaction vessel was compartmentized by a sintered glass partitionwhich was located immediately above the valved outlet. The reactionvessel was attached to a receiving flask, equipped with a vacuum outlet,through the valved outlet. To this reaction vessel were added 37.4 grams(0.203 mol) of potassium phthalimide in a mixture of 250 ml. of dioxaneand 500 ml. of diphenyl ether. Then 20.3 grams (0.1 mol) ofterephthaloyl chloride were dissolved in 250 ml. of dioxane and thismixture slowly added to the reaction vessel with constant stirring. Thecontents of the reaction vessel were gradually heated to refluxtemperature (between 110° to 115° C.) and maintained at reflux for 1hour. At the end of this time the dioxane solvent component was slowlydistilled from the reaction mixture through a fractional distillationcolumn. After the dioxane was completely removed the reaction mixturewas maintained at the boiling point of the diphenyl ether (about 254°C.) for an additional 1 hour. The hot solution, containing suspendedpotassium chloride by-product, was then quickly filtered, at the boiland under vacuum, into the receiving flask. White crystals ofN,N'-terephthaloyl bis-phthalimide (TBP) product immediately formed inthe filtrate. When the filtrate had cooled to about 100° C. the whitecrystals were collected by filtration and dried under heat and vacuum.The dried crystals [Product (I) ] weighed 36.5 grams, representing ayield of 87.3 percent and had a melting point of 340° to 345° C. As thefiltrate cooled to about 60° C. more white crystals formed which werecollected and dried under heat and vacuum. These crystals [Product (11)]had a melting point of 330° to 336° C. The precipitate collected below60° C. [Product (III)] was also dried under heat and vacuum and itsmelting point found to be 315° to 325° C. A portion of Product (I) wasrecrystallized from diphenyl ether. The recrystallized material had amelting point of 354° to 357° C. A portion of this recrystallizedmaterial was then again recrystallized from diphenyl ether and themelting point of the material found to be 365° to 367° C. An analysis ofthis latter material gave 68.06 percent carbon, 2.85 percent hydrogenand 6.35 percent nitrogen. The calculated values for the compound havingthe above formula are 67.93 percent carbon, 2.85 percent hydrogen and6.60 percent nitrogen.

In view of the above analysis the product is believed to have thestructure ##STR1##which has the chemical name N,N'-terephthaloylbis-phthalimide. For convenience, in this specification this isabbreviated and referred to hereinafter as TBP.

A widely employed method of increasing the rate of polycondensation inpolyester forming processes is by the addition of a reactive materialcapable of interlinking polymer chains. Such ester interlinkingmaterials provide efficient and economical means for producing polymersof high molecular weight. In the preparation of poly(ethyleneterephthalate), for example, the use of aromatic ortho carbonates (U.S.Pat. No. 3,714,125), carbonate derivatives of monovalent phenols (U.S.Pat. No. 3,444,141) and diaryl esters of dicarboxylic acids (U.S. Pat.No. 3,433,770) are but a few of the materials that have been employed toincrease the rate of polycondensation to provide a high molecular weightpolyester in shorter than normal condensation reaction times. Thedevelopment of a new interlinking material is an important contributionto this field. The new product was tested and found to be an effectiveinterlinking agent.

The utility of the new high melting interlinking agent is shown in theexamples below.

EXAMPLES 2 - 5

In each of Examples 2 through 5 a glass reaction tube approximately 35centimeters long having an inside diameter of 38 millimeters, equippedwith a side arm, a nitrogen gas inlet tube and stirrer was charged with50 grams of dimethyl terephthalate, 40 grams of ethylene glycol and 0.01percent (calculated as metal) of manganese octoate. Nitrogen gas wasslowly passed into the reaction tube and over the mixtures. The mixturewas stirred and heated by means of a vapor bath, which surrounded thetube, having a temperature of 240° C. After completion of thetransesterification reaction, polycondensation was commenced by adding0.0123 gram (calculated as metal) of antimony trioxide, increasing thetemperature of the mixture to 280° C. and gradually reducing thepressure in the tube to 0.05 millimeter of mercury pressure. In Examples2 and 3 the polycondensation reactions were carried out for 50 minutesand the mixtures sampled to determine their intrinsic viscosities (I.V.)at that point. Varying amounts of the twice recrystallized TBP ofExample 1 were then added to these mixtures and the polycondensationreactions continued for an additional 2 minutes. Examples 4 and 5 werecarried out in the same manner except that the polycondensation reactionwas run for 65 minutes before sampling and addition of the twicerecrystallized TBP prepared in Example 1. All pertinent data in eachinstance were noted and recorded. These data are set forth in Table I.

                  TABLE I                                                         ______________________________________                                        Example                                                                              Polycondensation                                                                           Original Amount of                                                                              Final                                   No.    Time, Minutes                                                                              I.V.     TBP Added.sup.(a)                                                                      I.V.                                    ______________________________________                                        2      50           0.509    1.40     0.735                                   3      50           0.509    1.96     0.817                                   4      65           0.592    1.10     0.908                                   5      65           0.592    1.54     0.926                                   ______________________________________                                         .sup.(a) Parts by weight per 100 parts by weight of dimethyl                  terephthalate.                                                           

EXAMPLE 6

Employing the same apparatus, materials and amounts thereof as in any ofExamples 2 through 5 a comparative example was run in which no highmelting TBP was added in order to illustrate the normal polycondensationrate. During the polycondensation reaction samples were withdrawn every10 minutes. The results are set forth in Table II.

                  TABLE II                                                        ______________________________________                                        Sample No.                                                                              Polycondensation Time, Minutes                                                                     I.V.                                           ______________________________________                                        1         40                   0.435                                          2         50                   0.509                                          3         60                   0.565                                          4         70                   0.621                                          5         80                   0.666                                          6         90                   0.703                                          7         100                  0.735                                          8         110                  0.754                                          9         120                  0.762                                          ______________________________________                                    

From a comparison of Tables I and II it is apparent that the addition ofsmall amounts of pure TBP greatly accelerates the polycondensation rateover that of the normal polycondensation reaction rate.

EXAMPLES 7 - 12

In each of examples 7 - 12, 50 grams of poly (ethylene terephthalate)having an I.V. of 0.809 were charged to a glass reaction tubeapproximately 35 centimeters long having an inside diameter of 38millimeters and equipped with a side arm, nitrogen gas inlet tube andstainless steel stirrer. In each of the examples the poly(ethyleneterephthalate) was melted at 280° C. under a nitrogen atmosphere. Toeach of Examples 8 - 12 were added 0.5 part by weight of one of the TBPproducts from Example 1 per 100 parts by weight of polyester. Themixtures were stirred at constant speed, at 280° C. and under nitrogenatmosphere for five minutes and then discharged. The I.V. of each of thepolymer samples was again determined. The results from these experimentsare listed in Table III. These results demonstrate that only TBP havinga melting point of at least 330° C. possesses the ability to interlinkpolyester chain ends and that the higher the purity the greater thisability. These examples also demonstrate the ability of high melting TBPto cause further polymerization of high I.V. polyester under meltpolymerization conditions, a feat which normally would have to becarried out under solid state polymerization conditions.

                  TABLE III                                                       ______________________________________                                                               M.P. of TBP                                            Example No.                                                                             Original I.V.                                                                              Added, ° C.                                                                       Final I.V.                                  ______________________________________                                          7.sup.(a)                                                                             0.809        --         0.809                                        8        0.809        315-325    0.807                                        9        0.809        330-336    0.828                                       10        0.809        340-345    0.899                                       11        0.809        354-357    0.959                                       12        0.809        365-367    1.051                                       ______________________________________                                         .sup.(a) Control                                                         

It should be noted that TBP having a melting point below 330° C.,surprisingly, causes a decrease in the original I.V. of polyester asevidenced by the result obtained in Example No. 8.

EXAMPLES 13 - 15

In each of Examples 13 through 15, 30 pound quantities of commercialpoly(ethylene terephthalate) chips having an I.V. of 0.972 were added toa 3-cubic foot blender-dryer and tumbled for 2 hours at 150° C. under0.1 millimeter of mercury vacuum. In addition, Example 14 contained 0.4part by weight of the twice recrystallized TBP samples from Example 1per 100 parts by weight of polyester and Example 15 contain 0.4 part ofa TBP sample prepared in accordance with the teachings of U.S. Pat. No.2,558,675 (Example 5) per 100 parts by weight of polyester. Employing a1-inch extruder, the three samples were then separately extruded intofibers. The residence time of the resin in the extruder wasapproximately 1 minute. All pertinent physical data are listed in TableIV below.

                                      TABLE IV                                    __________________________________________________________________________                                    Loss in                                       Example                                                                            Original I.V.                                                                          Fiber I.V.                                                                           Fiber M.P. ° C.                                                                   I.V. %                                        __________________________________________________________________________    13   0.972    0.847  258.0      12.9                                          14   0.972    0.965  257.5       0.7                                          15   0.972    0.811  257.0      16.6                                          __________________________________________________________________________

From Example 14 it can be seen that the high melting TBP of the presentinvention functions most effectively to essentially retain in the fiberthe high molecular weight of the polyester feed resin, whereas the priorart TBP does not. And again, it was most surprising that thepoly(ethylene terephthalate) sample containing the prior art TBP(Example 15) experienced a greater loss in I.V. than the sample in whichno TBP had been added.

EXAMPLES 16 - 24

The procedure employed in Examples 13 - 15 was repeated in Examples 16 -24 employing varying amounts of the twice recrystallized TBP fromExample 1 and poly (ethylene terephthalate) resin of varying initialintrinsic viscosities. Employing a 1-inch extruder, all samples wereseparately extruded into fibers. All pertinent data are listed in TableV below.

                  TABLE V                                                         ______________________________________                                        Example           Amount of         Fiber.sup.(c)                             No.    Resin I.V. TBP Added.sup.(a)                                                                        Fiber I.V.                                                                           Tenacity g/d                              ______________________________________                                        16.sup.(b)                                                                           0.592      0          0.502  --                                        17     0.592      0.5        0.657  --                                        18     0.592      1.0        0.738  --                                        19     0.592      1.5        0.770  --                                        20     0.592      2.0        0.790  --                                        21.sup.(b)                                                                           0.627      0          0.583  6.07                                      22     0.627      1.5        0.806  7.90                                      23.sup.(b)                                                                           0.759      0          0.689  6.78                                      24     0.759      0.8        0.939  8.32                                      ______________________________________                                         .sup.(a) Parts by weight TBP per 100 parts by weight of polyester             .sup.(b) Controls wherein no TBP was blended with the polyester               .sup.(c) g/d - grams per denier                                          

The above examples illustrate the ability of the high melting TBP toproduce fibers of higher I.V. than the resin from which they wereformed. In the absence of the high melting TBP the fibers exhibit alower I.V. than the starting resin. This decrease in I.V. isattributable to the mechanical and thermal degradation which the resinundergoes, in extrusion or spinning apparatus, during its conversion tofibers.

EXAMPLES 25 - 26

In each of Examples 25 and 26, 70 grams of poly (tetramethyleneterephthalate) having an I.V. of 0.8 and 30 grams of poly(tetramethyleneisophthalate/azelate) having an I.V. of 0.75 were charged to a glassreaction tube approximately 35 millimeters long, having an insidediameter of 38 millimeters and equipped with a side arm, nitrogen gasinlet tube and stainless steel stirrer. The polyester mixture in Example25 was melted at 280° C. under a nitrogen atmosphere with constantstirring. Once the polyester mixture was completely melted, 1.5 parts byweight of the twice recrystallized, high melting TBP prepared in Example1 per 100 parts by weight of polyester were added and the reactionmixture stirred for five minutes. The I.V. of the resulting product was0.98.

The polyester mixture in Example 26 was also melted at 280° C. withconstant stirring, but under vacuum. Again, once the polyester mixturewas completely melted, 1.5 parts by weight of the twice recrystallizedTBP prepared in Example 1 per 100 parts by weight of polyester wereadded to the melt and stirred for 5 minutes. The I.V. of the resultingproduct was 1.05.

The utility of the new high melting TBP was shown above employingpoly(ethylene terephthalate) and a mixture of poly(tetramethyleneterephthalate and poly(tetramethylene isophthalate/azelate). The newhigh melting TBP can be also be employed in combination with otherpolyesters and copolyesters derived from various other dicarboxylicacids or lower alkyl esters thereof with various other glycols employingany of the well known polyester forming processes. Representativeexamples of other useful dicarboxylic acids include aromaticdicarboxylic acids such as isophthalic acid, 2,6- and 2,7-naphthanoicacid, p,p'-diphenyl dicarboxylic acid and the like; cycloaliphaticdicarboxylic acids such as hexahydroterephthalic acid and the like andaliphatic dicarboxylic acids such as adipic acid, suberic acid, azelaicacid, sebaci acid and the like. Mixtures of these acids can also beemployed. Representative examples of the C₁ to C₄ alkyl esters of theabove acids include the dimethyl, diethyl, dipropyl, diisopropyl,dibutyl and diisobutyl esters or mixtures thereof. Finally, in additionto ethylene glycol, glycols of the series HO(CH₂)_(n) OH wherein n is aninteger from 2 to 10, cycloaliphatic glycols and aromatic glycols can beused, examples of which include proplene glycol, tetramethylene glycol,neopentyl glycol, hexamethylene glycol, decamethylene glycol,cyclohexane dimethanol, di-β-hydroxyethoxy benzene and the like.Mixtures of these various glycols can also be employed. Because of itscommercial significance, however, poly(ethylene terephthalate) preparedfrom either terephthalic acid or dimethyl terephthalate and ethyleneglycol is the preferred polyester for use with the above described newhigh melting TBP.

The maximum molecular weight increase which can be achieved through theuse of the high melting TBP material is dependent on the number of molsof polymerizing polyester or polyester resin and on the hydroxyl endgroup concentration therein. To achieve the maximum molecular weightincrease nz/2 mols of the high melting TBP are required, where n is thenumber of mols of polymerizing polyester or polyester resin and x is thenumber of hydroxyl end groups present in each molecule of polyester.However, the high melting TBP can also be employed in varying amounts toachieve any molecular weight increase desired or to prevent anymolecular weight decrease, with the actual amount employed beingdependent on the purpose for which the high melting TBP is beingemployed, the intrinsic viscosity of the polymerizing polyester orpolyester resin at the time of addition and on the ultimate molecularweight desired.

In general, the amount of the high melting TBP will range from about0.01 to about 25.0 parts by weight per 100 parts by weight of either theoriginal dicarboxylic acid or lower alkyl ester thereof when the TBP isadded to a polymerizing melt of a polyester or the polyester resin whenthe TBP is added to remelted polyester resin, solid state polymerizingpolyester or to polyester feed resin during the extrusion or spinningthereof into fiber. Specifically, when adding the high melting TBP to amelt polymerizing polyester of at least 0.2 I.V., the amount of the TBPwill range from 1.0 to 20.0 parts by weight per 100 parts by weight ofthe starting dicarboxylic acid or lower alkyl ester thereof. Theseamounts provide for increased polymerization rates and the attainmentpolyesters having higher molecular weight in shorter than normal processtimes. When adding the high melting TBP to a melt polymerizing polyesterof at least 0.6 I.V. the amount added will range from 0.1 to 5.0 partsby weight per 100 parts by weight of the starting dicarboxylic acid orlower alkyl ester thereof to provide polyesters of even higher molecularweight. The amount of high melting TBP employed, when added to 0.6 I.V.polyester granules or chips for subsequent melt or solid statepolymerization or spinning, will also range from 0.1 to 5.0 parts byweight but will, in this instance, be based on 100 parts by weight ofthe polyester resin. Finally, when the high melting TBP is added tofiber forming polyester resin, having an I.V. of at least 0.8, theamount employed, depending on whether the objective is to produce afiber having the same or higher I.V. than the starting resin, will rangefrom 0.01 to 3.0 parts by weight per 100 parts by weight of thepolyester resin. Addition of the high melting TBP to fiber formingpolyester can be carried out either prior to or simultaneously with theaddition of the polyester resin to the extrusion or spinning apparatus.Either method will provide the desired results.

While certain representative embodiments and details have been shown forthe purpose of illustrating the invention it will be apparent to thoseskilled in this art that various changes and modifications may be madetherein without departing from the spirit or scope of the invention.

What I claim is:
 1. In a process for preparing high molecular weightlinear polyesters of an aromatic dicarboxylic acid selected from thegroup consisting of terephthalic acid, isophthalic acid, 2,6-naphthoicacid, 2,7-naphthlic acid and p,p'-diphenyl dicarboxylic acid andcopolyesters of at least one of said acids and a second acid by meltpolymerizing the reaction product of the esterification ortransesterification of the dicarboxylic acid or lower alkyl esterthereof with a glycol of the series HO(CH₂)_(n) OH where n is an integerranging from 2 to 10 the improvement which comprises adding to thepolymerizing product, when the intrinsic viscosity of said polymerizingproduct is at least 0.2, from 1.0 to 20.0 parts by weight ofN,N'-terephthaloyl bis-phthalimide, having a melting point of at least330° C., per 100 parts by weight of the dicarboxylic acid or lower alkylester thereof and reacting it with said polymerizing product to form ahigh molecular weight product.
 2. The improvement of claim 1 wherein theN,N'-terephthaloyl bis-phthalimide is added to the polymerizing productof the transesterification reaction of dimethyl terephthalate withethylene glycol.
 3. In a process for preparing high molecular weightlinear polyester and copolyesters having an intrinsic viscosity of atleast 0.6 the improvement which comprises adding to a polyester orcopolyester from 0.1 to 5.0 parts by weight of N,N-terephthaloylbis-phthalimide, having a melting point of at least 330° C., per 100parts by weight of said polyester or copolyester and reacting it withthe polyester or copolyester.
 4. The improvement of claim 3 wherein thepolyester is poly(ethylene terephthalate).
 5. In a process for preparinghigh molecular weight polyester and copolyester fibers from polyesterand copolyester resins by melt extrusion of resins, having an intrinsicviscosity of at least 0.60 as measured in a 60/40phenol/tetrachloroethane mixed solvent at 30° C. the improvement whichcomprises adding to a polyester or copolyester from 0.1 to 5.0 parts byweight of N,N'-terephthaloyl bis-phthalimide, having a melting point ofat least 330° C., per 100 parts by weight of said polyester orcopolyester and reacting it with the polyester or copolyester.
 6. Theimprovement of claim 5 wherein the polyester resin is poly(ethyleneterephthalate).